diff --git a/projects/clr/rocclr/device/pal/paldevice.cpp b/projects/clr/rocclr/device/pal/paldevice.cpp
index c6aa33fe94d2..ce65660fef73 100644
--- a/projects/clr/rocclr/device/pal/paldevice.cpp
+++ b/projects/clr/rocclr/device/pal/paldevice.cpp
@@ -352,14 +352,14 @@ void NullDevice::fillDeviceInfo(const Pal::DeviceProperties& palProp,
   info_.globalMemSize_ = (static_cast<uint64_t>(std::min(GPU_MAX_HEAP_SIZE, 100u)) *
                           static_cast<uint64_t>(localRAM) / 100u);
 
-  uint uswcPercentAvailable =
-      ((static_cast<uint64_t>(heaps[Pal::GpuHeapGartUswc].logicalSize) / Mi) > 1536 && IS_WINDOWS)
-          ? 75
-          : 50;
+  const uint64_t gartSize = static_cast<uint64_t>(heaps[Pal::GpuHeapGartUswc].logicalSize);
+
+  // The aperture is host DRAM the GPU addresses directly, so discount it by at most
+  // 25%, capped at 4 GiB, rather than the 50/75% heuristic. Applies to every APU.
+  const uint64_t gartCredit = gartSize - std::min<uint64_t>(gartSize / 4, 4 * Gi);
+
   if (settings().apuSystem_) {
-    info_.globalMemSize_ +=
-        (static_cast<uint64_t>(heaps[Pal::GpuHeapGartUswc].logicalSize) * uswcPercentAvailable) /
-        100;
+    info_.globalMemSize_ += gartCredit;
   }
 
   // Find the largest heap form FB memory
@@ -373,10 +373,10 @@ void NullDevice::fillDeviceInfo(const Pal::DeviceProperties& palProp,
 
 #if IS_WINDOWS
   if (settings().apuSystem_) {
-    info_.maxMemAllocSize_ = std::max(
-        (static_cast<uint64_t>(heaps[Pal::GpuHeapGartUswc].logicalSize) * uswcPercentAvailable) /
-            100,
-        info_.maxMemAllocSize_);
+    // One allocation is not confined to a single heap, so the ceiling is the whole pool:
+    // carve-out plus aperture. Measured on gfx1151 at 100 GiB across a carve-out split
+    // 64 GiB visible plus 32 GiB invisible.
+    info_.maxMemAllocSize_ = info_.globalMemSize_;
   }
 #endif
   info_.maxMemAllocSize_ =
diff --git a/projects/clr/rocclr/device/pal/palsettings.cpp b/projects/clr/rocclr/device/pal/palsettings.cpp
index b5d169aa1225..d28f8a87b61b 100644
--- a/projects/clr/rocclr/device/pal/palsettings.cpp
+++ b/projects/clr/rocclr/device/pal/palsettings.cpp
@@ -217,7 +217,9 @@ bool Settings::create(const Pal::DeviceProperties& palProp,
         supportDepthsRGB_ = true;
       }
       if (use64BitPtr_) {
-        maxAllocSize_ = 64ULL * Gi;
+        // Unified memory APUs address the carve-out and the aperture as one pool, which
+        // already exceeds 64 GiB on shipping parts.
+        maxAllocSize_ = 256ULL * Gi;
       } else {
         maxAllocSize_ = 3ULL * Gi;
       }